5.0 mpi backfiring through exhaust. Won’t start

[woodguy81]

Engine serial number is m367508
2005 5.0 mpi

Working on my new boat and the port engine is giving me trouble. Finally got gas to it and it sounds like it wants to start but only backfires through the exhaust periodically. My question is if this would be a possible symptom of spinning the motor the wrong way since I just changed the starter. The prior owner told me the counter rotation happens in the bravo drive but He only even had it a couple months before selling it to me. Any thoughts?

 

[itsathepete]

Some engines are reverse rotation and some do it in the drive. A way to tell for sure is to remove a rocker arm cover and watch the valves. if you turn the engine clockwise (RH) and the exhaust valve closes and intake valve opens immediately, then it's right hand. If the intake closes then exhaust open its LH. Firing order for a RH engine is 18436572. firing order for LH engine is 12756348. Distributor for a LH engine has a different gear that is driven by the cam to make it spin the same way (clockwise) as a RH engine so that is not a way to tell, but if the plug wires are on wrong you can those symptoms. You could put the drive in forward gear and turn the engine and see which way the prop turns as well.

 

[Scott06]

Engine serial number is m367508
2005 5.0 mpi

Working on my new boat and the port engine is giving me trouble. Finally got gas to it and it sounds like it wants to start but only backfires through the exhaust periodically. My question is if this would be a possible symptom of spinning the motor the wrong way since I just changed the starter. The prior owner told me the counter rotation happens in the bravo drive but He only even had it a couple months before selling it to me. Any thoughts?

merc never made any reverse rotation engines for sterndrives your 2005 would be a standard rotation engine, the drive will be counter rotating

 

[Faztbullet]

Build up is distributer cap..common problem for boats that stay in water as vent screen stops up

 

[woodguy81]

Build up is distributer cap..common problem for boats that stay in water as vent screen stops up

New cap and rotor. Actually tried 2 new sets so far. No change.

 

[Eric 3.7LX Alpha 1]

Are the spark plug wires on correctly? Are you sure your #1 plug is connected to the the #1 cylinder on the cap? You should verify the location of #1 on the cap. The previous owner may have pulled all the wires off at once and didn't get it correct.

Pull the spark plugs out, leaving the wires attached to the cap, rotate the engine in the normal direction with your finger over the #1 spark plug hole until you feel air pushing your finger off. Continue to rotate the engine over slowly until the timing mark on the harmonic balancer aligns with the timing tab. Now check to see what cylinder the rotor is pointing at under the cap. Put a piece of tape on the body of the distributor just below where the bottom of the cap would sit, and mark the center of the rotor on the tape with a marker. Put the cap back on and see which post the marker is closest to without going past it.

I don't know how timing is set on that engine, but others can help with that if need be.

 

[dubs283]

There is no procedure for setting the timing on a merc mpi with a 555 ecm. Guessing this is the setup based on provided info.

There is a procedure for properly installing the distributor but I don't feel we need to go there just yet.

OP: you need to verify good clean spark, firing order, and fuel pressure/quality

 

[alldodge]

The prior owner told me the counter rotation happens in the bravo drive but He only even had it a couple months before selling it to me

My guess is the PO knows something he's not saying, and the Bravo does rotate in either direction
What's your serial number?

 

[Scott06]

New cap and rotor. Actually tried 2 new sets so far. No change.

Firing order is correct?

 

[woodguy81]

Serial number is in original post. I’ll verify firing order. Previous owner has a history of buying boats to fix then not Being able to do the work and reselling.

 

[alldodge]

Woops
ok you have the additional boost pump

 

[woodguy81]

Boost pump is the second fuel pump before the water separating filter? Are both fuel pumps the same or different models?

 

[Scott06]

Serial number is in original post. I’ll verify firing order. Previous owner has a history of buying boats to fix then not Being able to do the work and reselling.

so you fixed a fuel supply issue now its backfiring ? Might be worth running a compression test to see if exhaust valves are seating

 

[alldodge]

No they are different. Boost pump is a low pressure pump at the fuel filter, and high pressure is under the motor port side

 

[Faztbullet]

Make sure the IAC is not stuck shut ..just had one with similar symptoms in a Summerset HB

 

[woodguy81]

What is the correct order for the wires on the cap? Th

 

[woodguy81]

Nice thing about 2 engines is I can swap parts. Lol. I’ll check out the iac

 

[dubs283]

What is the correct order for the wires on the cap? Th

1-8-4-3-6-5-7-2 standard sbc LH rotation engine.

Flat cap may have numbers cast into it but you need to ensure the #1 position on the cap is indeed firing at #1 compression stroke. Once set the cylinder numbers are the above mentioned order of firing

 

[alldodge]

What is the correct order for the wires on the cap?

The cap should have the ports numbered, and left bank on left (1-3-5-7)and right on right (2-4-6-8). Makes it easier then the round ones

Key ON pump should run for 2 seconds. Use a rag and push in on the schrader valve on fuel rail and see if fuel squirts out. If no gas, turn key ON/OFF a few times till fuel shows up

 

[QBhoy]

Hi
Stick some jump cables on that engines starter battery, from the other engine. If your battery is anything but brilliant, these won’t fire up. They will however, seem to crank over perfectly fine, but the starter is taking that much load away from everything else and the result is that it will pop and shudder trying to start…especially just the moment that you let go of the key, stopping it cranking. Then for a split second, power returns to the ignition that was otherwise taken away.
Worth just making sure it’s not just this. It certainly threw me, first time it happened.